Activity Coefficients at Infinite Dilution of Organic Compounds in Trihexyl(tetradecyl)phosphonium Bis(trifluoromethylsulfonyl)imide Using Inverse Gas Chromatography Anne-Laure Revelli, † Laura M. Sprunger, ‡ Jennifer Gibbs, ‡ William E. Acree, Jr., ‡ Gary A. Baker, § and Fabrice Mutelet* ,† Laboratoire de Thermodynamique des Milieux Polyphase´s, Nancy-Universite´, 1 rue Grandville, BP 20451 4001 Nancy, France, Department of Chemistry, 1155 Union Circle #305070, University of North Texas, Denton, Texas 76203-5017, and Chemical Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6110 Activity coefficients at infinite dilution γ ∞ of organic compounds in the ionic liquid (IL) trihexyl(tetrade- cyl)phosphonium bis(trifluoromethylsulfonyl)imide were determined using inverse gas chromatography at three temperatures, T ) (302.45, 322.35, and 342.45) K. Linear free energy relationship (LFER) correlations have been obtained for describing the gas-to-IL and water-to-IL partition coefficients. Introduction Ionic liquids (ILs) are commonly defined as substances composed only by ions which are liquid at or close to room temperature. ILs are emerging as alternative green solvents in diverse applications. 1-7 Despite that their properties are not yet deeply understood, publications usually highlight their negligible vapor pressure, thermal and chemical stability, and the pos- sibility to design their physicochemical properties by suitable choice of the anion and the cation. Because of their negligible vapor pressure, ILs are claimed as “green” alternatives for volatile organic solvents (VOCs). Knowledge of properties of ILs and their mixtures is important for design and process synthesis. Activity coefficients at infinite dilution γ i ∞ are key pieces of information for the realization of many separation processes that employ ionic liquids. For example, the selectivity between components i and j in an ionic liquid S i,j ∞ (S i,j ∞ ) (γ i ∞ )/(γ j ∞ )) for the separation process can directly be derived from such data. The activity coefficient gives also information on the interactions between the solute and the ionic liquid. Recently, this thermodynamic property was predicted using the COSMO-RS method. 8-10 Eicke et al. 11 demonstrated that QSPR methods have a very good ability to predict values of ln γ i ∞ and predict solute-solvent interactions for various solutes in IL solvents. To quantify intermolecular solute-IL interactions, Acree, Abraham, and co-workers re- ported mathematical correlations based on the general Abraham solvation parameter model for the gas-to-solvent, K, and water- to-solvent, P, partition coefficients. 12-14 Recently, Sprunger et al. 15,16 modified the Abraham solvation parameter model log K)c cation +c anion +(e cation +e anion ) · E+(s cation +s anion ) · S+ (a cation +a anion ) · A+(b cation +b anion ) · B+(l cation +l anion ) · L (1) log P)c cation +c anion +(s cation +s anion ) · S+(a cation +a anion ) · A+ (b cation +b anion ) · B+(e cation +e anion ) · E+(V cation +V anion ) · V (2) by rewriting each of the six solvent equation coefficients as a summation of their respective cation and anion contribution. The dependent variables in eqs 1 and 2 are solute descriptors as follows: E and S refer to the excess molar refraction in units of (cm 3 · mol -1 )/10 and dipolarity/polarizability descriptors of the solute, respectively; A and B are measures of the solute hydrogen-bond acidity and basicity; V is the McGowan volume in units of (cm 3 · mol -1 )/100; and L is the logarithm of the gas- to-hexadecane partition coefficient at 298 K. Sprunger et al. calculated equation coefficients for 8 cations and 4 anions using a database that contained 584 experimental log K and 571 experimental log P values. No loss in predictive accuracy was observed by separating the equation coefficients into individual cation-specific and anion-specific values. The major advantage of splitting the equation coefficients into individual cation- specific and anion-specific contributions is that one can make predictions for more ILs. In this work, activity coefficients at infinite dilution of 39 polar and nonpolar compounds (alkanes, alkenes, alkynes, cycloalkanes, aromatics, alcohols) have been determined in the ionic liquid trihexyl(tetradecyl)phosphonium bis(trifluorometh- ylsulfonyl)imide at T ) (302.45, 322.35, and 342.45) K by gas-liquid chromatography. This ionic liquid has already been studied in the literature. However, it can be found two literature sources 17,18 for the thermodynamic properties of organic compounds in this phosphonium ionic liquid. These data show a large discrepancy. The modified LFER model proposed by Sprunger and co-workers was used to characterize the nature of solute interactions with this ionic liquid. The cation-specific coefficients were estimated using the partition coefficients of 45 organic compounds calculated at 298.15 K. Experimental Procedures and Results Materials or Chemicals. The ionic liquid trihexyl(tetrade- cyl)phosphonium bis(trifluoromethylsulfonyl)imide, [P 14,666 + ]- [NTf 2 - ], was prepared and dried following recently reported methods. 19 The ionic liquid was further dried under vacuum * Corresponding author. E-mail: mutelet@ensic.inpl-nancy.fr. Telephone number: +33 3 83 17 51 31. Fax number: +33 3 83 17 51 52. † Nancy-Universite´. ‡ University of North Texas. § Oak Ridge National Laboratory. J. Chem. Eng. Data 2009, 54, 977–985 977 10.1021/je800754w CCC: $40.75  2009 American Chemical Society Published on Web 02/02/2009